The present invention relates to a developer removing device for a copying apparatus, and more particularly to a developer removing device for a copying apparatus using a developer mix of magnetic carriers of small diameter and insulating non-magnetic toner to remove said magnetic carriers adhering to background or non-imaged portion of an image bearing member such as a photosensitive member and a dielectric paper.
As well known in the art, a two-component developing material has hitherto been employed for developing an electrostatic latent image. The two-component developing material comprises in general a mixture of toner particles including synthetic resin and a coloring agent such as carbon black and magnetic carriers including powdery iron. When in use, toner and carriers are thoroughly mixed and agitated to triboelectrically charge the toner to a polarity opposite to that of the electrostatic latent image and then adhered onto the image portion of a latent image by a magnetic brush developing method to develop the same.
Said carriers hitherto used generally have an average particle diameter of about 50 to 200 microns and this is quite large as compared with the average particle diameter of said toner which is only about 10 microns. Accordingly, in the development using such carriers of large diameter, bristles of the magnetic brush tend to become strong or hard and this causes harsh quality of the image and poor contrast as well as fogging on image. In addition, it is difficult to suitably maintain the mixing ratio of toner and carriers since only toner is consumed.
Another developing method using a mono-component developing material is also well known. This mono-component developing material is generally employed in the form of a mass of magnetic toner particles each being constituted by a synthetic resin block containing magnetic particles, uniformly dispersed therein, and coated with an electroconductive material such as carbon black. The development of the electrostatic latent image into the toner image according to this method is performed by way of magnetic brush development technique as is the case with the toner image development using the two-component developing material. Whereas in the toner image development using the two-component developing material the electrostatic attractive force acting between the toner, which has acquired an electrical charge as a result of tribo-electricity, and the electrical charges of the latent image on the photoconductive support surface plays a major role in transfering toner particles onto an image bearing surface to form the toner image thereon, a similar transfer in the toner image development using the mono-component developing material takes place by the combined effect of a force of electrostatic attraction, exerted between the electric charges of the latent image and the charges which have been injected, with a polarity opposite to that of the latent image, through an electroconductive sleeve or shell into the magnetic toner particles as the latter approaches the latent image on the image bearing member, the value of the electric charges so injected corresponding to that of the latent image, and a force of magnetic attraction exerted by a magnet positioned internally of the sleeve or shell for magnetically retaining the magnetic toner particles on the sheeve or shell.
The toner image development using the mono-component developing material substantially eliminates disadvantages inherent in the toner image development using the two-component developing material resulting from the inclusion of the carrier which forms the unconsumable part of the two-component developing material, but it has some disadvantages, for example, the lack of a high fidelity reproduction in gradation, difficulty in fixing and inability to use an ordinary plain copying paper because of the difficulty involved in transferring the toner image from a photoconductive support surface to such plain copying paper by the use of a corona discharge technique. These disadvantages are considered to originate from the fact that the mono-component developing material, i.e., the magnetic toner, is required to have a relatively low resistance to facilitate the charge injection from the photoconductive support surface to the magnetic toner through the sleeve or shell during the application of magnetic toner particles onto the electrostatic latent image on the photoconductive support surface. Because of the requirement that the mono-component developing material have a relatively low electric resistance, the toner image development using the mono-component developing material is likely to involve instability of transfer of the toner image from the photoconductive support surface to the sheet of final support material which may result in insufficient transfer of the toner image onto the sheet of final support material to every detail and/or adherence of magnetic toner particles to non-image areas, i.e., background deposition of the magnetic toner particles. The consequence is that the image reproduced on the sheet of final support material after the toner image transferred onto the sheet of final support material has been fixed will be blurred and/or foggy.
In order to eliminate the above described disadvantages and inconveniences inherent in the two-component developing material and the mono-component developing material, a developing method using a mixture of magnetic carriers of small diameter and insulative non-magnetic toner has been proposed by the assignee of this application in Japanese Laid Open Patent Application SHO 54-66134 or in the copending U.S. patent application Ser. No. 104,456 filed on Dec. 17, 1979 now U.S. Pat. No. 4,284,702 which is continuation of application Ser. No. 949,426 filed on Oct. 5, 1978.
In this method, magnetic carriers of substantially the same size as the toner are used. By mixing, the carriers and toner are tribo-electrically charged to a polarity opposite to one another and by frictionally contacting them with the electrostatic latent image formed on the image bearing member by the magnetic brush developing method, toner is adhered or transferred onto the image portion of the latent image. With this method, an image of excellent contrast and high quality will be obtained while substantially eliminating the afore-described drawbacks.
In the magnetic brush developing method using said magnetic carriers of small diameter and non-magnetic toner, a biasing voltage of the same polarity as the electrostatic latent image is applied between a developing means and the image bearing member in order to prevent fogging in the background or non-imaged portion of the latent image. However, since its biasing potential is greater than the potential on the background portion, the magnetic carriers charged to the same polarity as the latent image adhere to the background portion. Adherences of these magnetic carriers to the background area are particularly notable in the area surrounding the image portion of the latent image due to the presence of an electric field.
The magnetic carriers of small diameter adhered to the background portion will not by themselves transfer onto a copying paper during the image transfer in a toner image transfer type copying apparatus because they are charged to a polarity the same as the electrostatic latent image. Instead, these carriers present between the photosensitive member and the copying paper will provide a larger air gap therebetween to reduce electrostatic attraction of the copying paper and this makes the removal of copying paper from the photosensitive member quite simple. While the adherences of said magnetic carriers to the non-imaged portion will bring about the aforedescribed advantages, on the other hand it causes damage to the photosensitive member and a residual toner removing means. To be specific, the magnetic carriers include therein magnetic material of a hard nature and when these carriers intervene between the photosensitive member and the residual toner removing means, these parts become damaged which causes deterioration of the copying quality. This phenomenon becomes particularly notable when an elastic blade in contact with the surface of photosensitive member is used as the residual toner removing means. Accordingly, it becomes necessary to replace the photosensitive member as well as the residual toner removing means more frequently than is desirable. Thus, there is a need for certain measures to effectively remove the magnetic carriers of small diameter adhered to the background portion.